28 THE DATA OF BIOLOGY. 



which the tissues ah-eady contain. Hence, to the extent that 

 such escape is facihtated by evaporation, and this evaporation 

 facilitated by heat, lieat becomes an agent of re-distribution 

 in the animal organism. 



§ 13. Light, which is now known to modify many inor- 

 ganic compounds — whicli works those chemical changes 

 utilized in photography, causes the combination of certain 

 gases, alters the molecular arrangements of many crystals, 

 and leaves traces of its action even on substances that are ex- 

 tremely stable, — may be expected to produce marked eifects 

 on substances so complex and unstable as those which make 

 up organic bodies. It does produce such marked effects ; 

 and some of them are among the most important that 

 organic matter undergoes. 



The molecular changes wrought by light in animals, are 

 but of secondary moment. There is the darkening of the 

 skin that follows exposure to the sun's rays. There are 

 those alterations in the retina which cause in us sensations 

 of colours. And on certain eyeless creatures that are semi- 

 transparent, the light permeating their substance works 

 some effect evinced by movement. But speaking generally, 

 the opacity of animals limits the action of light to their 

 surfaces ; and so renders its direct physiological influence 

 but small.* On plants, however, the solar rays that 



produce in us the impression of yellow, are the immediate 

 agents of those molecular changes through which are hourly 

 accumulated the materials for further growth. Experiments 

 have shown that when the sun shines on living leaves, they 

 begin to exhale oxygen and to accumulate carbon and 

 hydrogen — results which are traced to the decomposition by 

 the solar rays, of the carbonic acid and water absorbed. 

 It is now an accepted conclusion that, by the help of certain 



* The increase oi respiration found to result from the presence of light, is 

 probably an indirect effect, xt is most likely auc to the receotiou of more vivid 

 impressions through the eyes, and to the consequent nervous stimulation. 



